After crude oil is produced from an offshore oil production platform in an oil field, there are two options to deal with the produced oil: 1) export the oil out through a subsea pipeline system; or 2) store the oil in a temporary storage facility, which could be a fixed part of the platform itself or a separate storage facility, and then transport the oil out via an oil shuttle tanker. If the production of the field in terms of quantity or duration is insufficient to justify the cost of a subsea pipeline connecting the platform to the shore, a platform with a safe, reliable and low cost temporary oil storage system must be provided for the development of such a field.
The location of a temporary oil storage system is an important issue. There are only three possible options: 1) build the system above water as a fixed part of the offshore platform's topsides; 2) construct the system at water surface as a part of a floating structure; or 3) build the system under water as a part of the platform substructure or as a separate facility which is usually connected to the platform topsides via pipes for oil loading and offloading operations. Each of these options has pros and cons during its applications. With Option 1, the oil tank is usually built as a fixed part of the topsides commonly beneath the process modules and living quarters. This arrangement makes the tank independent of waves and current-induced environmental loadings. However, placing oil compartments beneath the process modules and living quarters could constitute a serious safety hazards. An even more critical issue is that this arrangement places a large mass at the platform top with a large weight variation during a loading and offloading operation, which could greatly influence the dynamic characteristics of the platform. Therefore, this option has been only used for very shallow water fixed platform applications so far in the offshore industry. With Option 2, it is easy to increase the oil storage capacity of a floating structure such as a ship-shaped FPSO vessel. However, the cost of a floating structure is usually much higher than a conventional fixed platform because the floating structure is subject to a large wind and wave induced environmental loading and it requires an expensive station keeping system to keep the floating structure in location. Therefore, floating structures are usually applied in deep water applications or some shallow water applications where a subsea pipeline system becomes too expensive to be feasible. With Option 3, it is the most preferred method for both offshore fixed platforms and offshore floating structures because the wind induced environmental loading is totally eliminated and the wave induced environmental loading should also be significantly reduced, if the top of the storage tank is sufficiently submerged under water surface. In addition, the platform's vertical center of gravity (COG) should also be lowered, which generally benefits the platform's dynamic characteristics as well as the motion responses. However, a subsea oil storage system usually faces many challenges under different types of applications. One of the most critical challenges is how to handle a large amount of buoyancy and related weight variations during the system's loading and offloading operations.
For a submerged offshore storage system, there are two common methods to store oil or other hydrocarbon liquids underwater. The first one is called “wet storage method”. Under this method, oil and seawater are stored together underwater within the same tank. Because of the density difference (crude oil density: ˜0.8 to ˜0.9), oil or other non-water solvable hydrocarbon liquids should stay at the top of the tank above the seawater and maintain a physical contact surface between the two types of liquids. During a loading operation, an amount of produced oil is imported into the tank and the equivalent amount of seawater, in the volume term, is then displaced to keep the total volume of oil and sea water constant within the tank. During an offloading operation, the process is reversed: seawater is imported into the tank to displace the oil and to keep the total volume constant inside the tank. The first advantage of the wet storage method is that the on-bottom weight at the storage tank bottom should have limited variations, generally only about 15% reduction of the maximum on-bottom weight at the tank bottom between loading and offloading operations. The second advantage of the wet storage method is that the tank does not need to be designed as a pressured vessel because the tank internal pressure is hydrostatically balanced with outside seawater throughout its service life. However, the wet storage method also possesses three major disadvantages. The first disadvantage is the environmental pollution concern. Under the dynamic motions, the contact surface between oil and seawater could produce a mixed layer to cause environmental pollution if discharged out to the sea. The second disadvantage of the wet storage method is the issue of thermal insulation. It is very hard to keep oil warm with the existence of a large contact surface between warm oil and cold seawater. The third disadvantage is that it cannot store water-soluble liquids such as methanol. Because of these critical disadvantages, especially with the environmental pollution concerns, this wet storage method has had very few field applications so far, either for fixed platforms or for floating platforms, in spite of the tremendous efforts made in the offshore industry.
The second common method to store oil or other hydrocarbon liquids underwater is called “dry storage method”. Under this method, oil or other hydrocarbon liquids can be simply stored in an underwater storage tank without any contact with sea water. In addition, the underwater storage tank can be easily applied with a good thermal insulation protection to the stored oil. However, this simple dry storage method faces two critical challenges during its service life. The first challenge is the large buoyancy issue due to the emptied room of the tank during an oil offloading process. The second challenge is the inert gas induced environmental pollution concern. In order to prevent the evaporated gas from escaping the oil storage tank to pollute outside air, inert gas is commonly used to be injected into a closed oil storage tank's top part above the oil surface. However, the required inert gas operations such as generating, blanketing and venting could become another source of pollution hazards. To overcome the first challenge of excessive buoyancy, common practice is to use a concrete gravity based platform. The heavy weight of a concrete structure helps to offset the extra buoyancy generated by an oil offloading operation. Another solution for the first challenge is to build extra ballast tanks and to ballast water in during an offloading operation in order to compensate for the discharged oil. Due to the above concerns, so far there have been only a very limited number of field applications, mostly in the form of concrete gravity based platforms, in the offshore oil & gas industry.
One specific example of the “dry storage method” was illustrated by Wu in his U.S. Pat. No. 8,292,546. In this example, one liquid storage compartment is coupled with one water ballast compartment under a symmetrical arrangement with the existence of pressured inert gas above both liquid surface and the water surface in these two compartments. During oil loading and offloading operations, the system functions as an equal mass flow rate displacement system with a pair of coupled pumps by which a constant system mass is maintained and the COG is moved only along a vertical axis. The primary advantage of this system is that the whole system weight is constantly maintained during loading and offloading operations. However, this system gives rise to three concerns. Firstly, the oil storage volume utilization is low. More than 50% of the total system volume cannot be utilized for oil storage, but has to be left for containing seawater and inert gas. Therefore, a large amount of buoyancy will be still produced during oil loading and offloading operations (more than 50% in contrast to only about 15% in the “wet storage method” system). To overcome this buoyancy issue, a heavy concrete structure usually has to be utilized, instead of using steels which are commonly used for offshore platform constructions. Secondly, both liquid storage compartment and water compartment have to be designed as pressured vessels because of the existence of inert gas. Thirdly, the system operation is heavily dependent on a complex system using a pair of coupled pumps for oil and for water separately. This arrangement could become a safety concern during the oil storage system operation.
Therefore, there is a need for a submerged offshore storage system that combines wet storage method and dry storage method, and maintains advantages and overcomes disadvantages of these two storage methods.